Longhorn Dam Labyrinth Weir Conceptual Evaluation

LonghornDamLabyrinthWeir

ConceptualEvaluation

Preparedfor:

Austin Energy

June2013

Preparedby:

FREESEANDNICHOLS,INC.10814JollyvilleRd.,Bldg.4,Ste100

Austin,Texas78759512‐617‐3137

TBPEFirmF‐2144


ConceptualEvaluationPreparedfor:

Austin Energy

June2013

Preparedby:

FREESEANDNICHOLS,INC.10814JollyvilleRd.,Bldg.4,Ste100

Austin,Texas78759512‐617‐3137

TBPEFirmF‐2144

AU412421

DRAFT

This document is released for the purpose of interim review under the authority of Dustin Mortensen, P.E., Texas No. 100000 on June 21, 2013. It is not to be used for construction, bidding or permit purposes. Freese and Nichols, Inc. Texas Registered Engineering Firm F‐ 2144

Longhorn Dam Labyrinth Weir Conceptual Evaluation Austin Energy

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TABLEOFCONTENTS

1.0 PROJECTOVERVIEW..................................................................................................................................1

1.1 Background.................................................................................................................................................1 1.2 Purpose.........................................................................................................................................................1 1.3 RiverFlowRates.......................................................................................................................................1

2.0 GATEDSTRUCTURE....................................................................................................................................4

2.1 OriginalDesign..........................................................................................................................................4 2.2 GateOperation...........................................................................................................................................6 2.3 GatedSpillwayRatingCurve...............................................................................................................9 2.4 HistoricalPerformanceofExistingDam.....................................................................................10 2.5 GateRehabilitationProjectSummary..........................................................................................12

3.0 FIXEDWEIR.................................................................................................................................................14

3.1 FixedWeirOverview...........................................................................................................................14 3.2 LabyrinthWeirExamples..................................................................................................................15 3.3 ProposedLabyrinthConfigurationatLonghornDam..........................................................17 3.4 LabyrinthWeirRatingCurve...........................................................................................................19 3.5 LabyrinthPerformance......................................................................................................................21 3.6 BackwaterAnalysis..............................................................................................................................25

4.0 LABYRINTHWEIRASSESSMENT.......................................................................................................26

4.1 LakeLevels...............................................................................................................................................26 4.2 HikeandBikeTrail...............................................................................................................................26 4.3 CesarChavezStreetatLamarBoulevard....................................................................................26 4.4 ExistingStructuresandthe100‐YearFloodplain...................................................................27 4.5 DebrisandSedimentation.................................................................................................................27

5.0 STAKEHOLDERMEETINGS...................................................................................................................29

5.1 PublicWorks...........................................................................................................................................29 5.2 ParksandRecreation...........................................................................................................................29 5.3 WatershedProtectionDepartment...............................................................................................30 5.4 AustinWaterUtility.............................................................................................................................31 5.5 LowerColoradoRiverAuthority....................................................................................................31 5.6 LakeBrazosDamSiteVisit................................................................................................................31

6.0 ENVIRONMENTALANDPERMITTINGCONSIDERATIONS.....................................................32

6.1 EnvironmentalPermitting................................................................................................................32 6.2 TCEQDamSafety...................................................................................................................................32


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6.3 StateHistoricPreservationOffice..................................................................................................32

7.0 OPINIONOFPROBABLECONSTRUCTIONANDOPERATIONCOSTS.................................34

7.1 GateRehabilitationProject...............................................................................................................34 7.2 LabyrinthWeirProject.......................................................................................................................34 7.3 LifeCycleCosts.......................................................................................................................................35

8.0 CONCLUSION...............................................................................................................................................36

8.1 Comparison..............................................................................................................................................36 8.2 Conclusion................................................................................................................................................37

TableofFigures

Figure1:LonghornDamExistingSitePlan...................................................................................................5 Figure2:CrossSectionofDamandStillingBasinthroughLiftGate.................................................6 Figure3:Downstreamsideofautomatedbasculegate...........................................................................7 Figure4:Downstreamviewofliftgate3.......................................................................................................8 Figure5:GateoperationoccursonthissidewalksharedwiththeHikeandBikeTrail...........9 Figure6:GatedSpillwayRatingCurve–BestCaseScenario.............................................................10 Figure7:HistoricalNormalLevelswithReleasesfromTwoTurbinesatTomMillerDam.11 Figure8:HistoricalLadyBirdLakeLevelscomparedtoBestCaseGateOperation................12 Figure9:PlanViewofTypicalLabyrinthWeir........................................................................................14 Figure10:LakeBrazosDamshortlyafterconstruction.......................................................................15 Figure11:LakeTownsendLabyrinthWeirunderConstruction(Schnabel,2013).................16 Figure12:ElmendorfDam................................................................................................................................16 Figure13:RenderingofLabyrinthWeiratLonghornDam................................................................17 Figure14:LabyrinthWeiratLonghornDamwithcyclesconnectedtoexistingbridgepiers.......................................................................................................................................................................................18 Figure15:LabyrinthWeiratLonghornDamwithcyclesnotconnected toexistingbridgepiers.............................................................................................................................................................................18 Figure16:RatingCurveforGatedStructureandLabyrinthWeir...................................................19 Figure17:HistoricalLadyBirdLakeLevelsvs.PredictedRatingCurves....................................20 Figure18:FractalLabyrinthWeir.................................................................................................................21 Figure19:HistoricalDamPerformanceandPredictedLabyrinthPerformance......................22 Figure20:ObservedandPredictedLakeLevelsforNovember2004Flood...............................23 Figure21:ObservedandPredictedLakeLevelsforSeptember2010Flood(TropicalStormHermine)...................................................................................................................................................................23 Figure 22: Observed and Predicted Lake Levels during typical LCRA Releases from OneTurbine.......................................................................................................................................................................24 Figure 23: Observed and Predicted Lake Levels during typical LCRA Releases from TwoTurbines....................................................................................................................................................................24


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TableofTablesTable1:DischargeforVariousStormEvents..............................................................................................2 Table2:HistoricalEventswithDischargeGreater than25,000cfs(USGSGage08158000,ColoradoRvatAustin)...........................................................................................................................................3 Table3:KeyFeaturesofGateRehabilitationConceptualDesign....................................................13 Table4:LakeLevelsatVariousStormEvents..........................................................................................20 Table5:GatedStructureRepair,OperationandMaintenanceItemCosts...................................34 Table6:LabyrinthWeirConstructionandOperationCosts..............................................................35 Table7:LifeCycleCosts.....................................................................................................................................35 Table8:AttributesofAlternatives.................................................................................................................36 Appendices Appendix A ‐ Backwater Figures Appendix B ‐ Flood Prone Areas Identified by Parks and Recreation Appendix C – Labyrinth Weir Opinion of Probable Construction Cost Appendix D ‐ Survey Points Appendix E ‐ Rating Curves Appendix F – Project Schedules


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EXECUTIVESUMMARY

Longhorn Dam was constructed with a gated spillway in 1960 to create a cooling pond for Holly Power

Plant. Austin Energy has operated and maintained the dam since construction. Now, with the

decommissioning of Holly Power Plant, Longhorn Dam will have no functional relationship to Austin

Energy and power generation. Nevertheless, Austin Energy continues to operate and maintain the dam

and gates. The dam is now in need of numerous repairs and upgrades to improve the reliability of the

dam and spillway gates. A conceptual design to repair the dam, replace the gate operators and make

improvements to the channel and river banks was previously prepared by Freese and Nichols, Inc. (FNI).

Austin Energy prioritized the repairs in the conceptual design and decided that $10 million in repairs

were dam operation and maintenance priorities. Prior to designing the proposed improvements, Austin

Energy requested that FNI evaluate the option to construct a fixed weir at Longhorn Dam in lieu of

repairing the gates and operators. It was anticipated that a fixed weir would reduce the operations and

maintenance requirements of the dam.

This report presents the conceptual evaluation of a labyrinth spillway as a fixed weir and replacement to

the gates at Longhorn Dam. A labyrinth spillway is a passive structure that uses a trapezoidal‐shaped

weir wall geometry (plan view) to increase the weir length in a channel and thus increase discharge. The

labyrinth weir has benefits over the gated structure; these benefits are directly related to the

elimination of the gates. The largest benefit is that the labyrinth does not require personnel to operate

the lift gates during storm events or to verify that the automated bascule gates are functioning properly.

Another benefit is predictable lake levels during storm events. The historical record indicates that lake

levels have risen several feet higher than predicted by river models due to spillway gates not being

timely opened. Operating spillway gates during storm events to maintain the desired lake levels is

challenging and requires frequent gate adjustment. The labyrinth weir will eliminate higher than

anticipated lake levels caused by difficulties in proper gate operation. The labyrinth weir will also

prevent water from being wasted, which has occurred when the gates release more water than planned

causing the lake level to drop below normal pool.

There are drawbacks to the labyrinth weir at Longhorn Dam. During major floods similar to those that

occurred in 1997 and 2002, the labyrinth weir will cause the lake level to be approximately two feet

higher than it would be if the spillway gates were operated properly and timely. Historically the

Longhorn Dam gated system has not approached this level of control. Another drawback of the labyrinth

weir is that debris will not flow over the dam as easily as it does through the gates. The labyrinth weir


ES‐2

will likely require debris removal, especially following storm events, although this could be mitigated

with a debris boom upstream.

The opinion of probable construction cost for the labyrinth weir is $15.0 million and the cost to repair

the gated structure is $10 million. Based on a 50‐year design life and 4% interest rate, the present value

of the gated structure is estimated at $20,000,000 and the present value of the labyrinth at

$17,040,000.

Longhorn Dam is a very important asset in need of improvements to properly serve its recreational

purpose. The labyrinth weir is a feasible replacement for the gated spillway. The selection of this

alternative over the gate repairs must consider if the advantages of reduced operational costs and more

predictable lake levels outweigh the disadvantages of higher lake levels during flood events and debris

removal.


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1.0 PROJECTOVERVIEW

1.1 BACKGROUND

Longhorn Dam is owned by Austin Energy and creates Lady Bird Lake. The dam was constructed in

conjunction with the Holly Power Plant to create a cooling pond for the plant. Now that the plant has

been de‐commissioned, Austin Energy is in the position of maintaining the dam solely for the purpose of

creating Lady Bird Lake. Austin Energy is charged with operating and maintaining the nine spillway gates

at the dam which are used to regulate the lake level and pass flood flows. The gates have a history of

poor operation and are not reliable. In 2012, Freese and Nichols (FNI) completed a conceptual design to

rehabilitate the dam and make improvements to the gate operation (Freese and Nichols 2012). The

proposed gate improvements would result in a dam that is more reliable and easier to operate.

The dam was previously operated and maintained by personnel at the Holly Plant. Because the Holly

Plant has ceased operation and is no longer staffed by Austin Energy personnel, ongoing efforts of gate

operations and maintenance at Longhorn Dam have been complicated. In an effort to minimize or

eliminate the need for gate maintenance and operations, Austin Energy requested that FNI conduct a

feasibility study of a fixed weir at Longhorn Dam as an alternative to the currently planned modifications

to the existing gates and hoists.

1.2 PURPOSE

This study developed a conceptual design of a fixed weir at Longhorn Dam and evaluated the feasibility

of the concept. The purpose was to evaluate lake levels with the labyrinth weir, identify stakeholders

who would be affected and develop budgetary opinions of probable cost. The conceptual design is

preliminary and intended to provide information to help decide how to proceed with improvements to

Longhorn Dam. The purpose of this report is to provide a summary of the concept and highlight

attributes and design considerations.

1.3 RIVERFLOWRATES

Daily flows through Longhorn Dam are largely controlled by Lower Colorado River Authority (LCRA)

releases from the Highland Lakes dams and especially Tom Miller Dam. There are two turbines at Tom

Miller Dam through which LCRA typically releases water. During storm events, flow through Longhorn

Dam comes from releases from Tom Miller Dam flood gates as well as flood flows from area creeks.


2

LCRA has estimated flow rates at Longhorn Dam due to various storm events (Halff Associates 2003).

Discharge from the turbines as well as the various storm events are shown in Table 1.

Table 1: Discharge for Various Storm Events

Event Discharge

(cfs)

One Turbine 1,750

Two Turbines 3,500

2‐year 14,564

5‐year 24,567

10‐year 29,945

25‐year 50,050

50‐year 90,046

100‐year 90,361

500‐year 366,912

Lake Travis and Mansfield Dam significantly reduce flooding in the City of Austin by storing floodwaters

in the lake and releasing them in a controlled manner. LCRA releases from Mansfield Dam are regulated

by the U.S. Army Corps of Engineers (USACE). The regulations typically require LCRA to limit flows near

Longhorn Dam to 30,000 cfs. Releases of floodwater from Lake Travis may last from a few days to a few

weeks. Historical records show that average daily flow rates due to releases from Mansfield Dam have

exceeded 25,000 cfs for extended periods on a number of occasions. Table 2 shows that there have

been 141 days where the average daily flow rate was above 25,000 cfs since Mansfield Dam was

completed in 1942. These flows occurred in nine different years. The 2010 storm is not included in Table

2 because the average daily flow in 2010 only reached 15,900 cfs according to the USGS.


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Table 2: Historical Events with Discharge Greater than 25,000 cfs (USGS Gage 08158000, Colorado Rv at Austin)

Year Total Days

Average Daily Flow Rate

Maximum Average Daily Flow Rate

1957 32 34,072 37,900

1959 4 31,125 35,400

1977 4 30,200 31,500

1987 11 27,291 31,700

1992 49 29,635 35,500

1997 16 27,400 28,500

2002 6 26,166 26,800

2004 4 26,450 27,100

2007 15 25,953 26,700 Flood events that began in December and ended after January 1 are included with the following year.


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2.0 GATEDSTRUCTURE

2.1 ORIGINALDESIGN

Longhorn Dam is located on the Colorado River in the City of Austin approximately 1.5 miles east of IH‐

35. The dam is approximately 1,240 feet long and consists of a 506‐foot wide gated spillway with an

earthen embankment on each side of the spillway. The gated spillway supports a four lane bridge for

Pleasant Valley Road and the Hike and Bike Trail. Figure 1 shows a plan view of the dam.

Longhorn Dam was constructed in 1959 to create a cooling water supply for the Holly Power Plant. The

dam also improved the reliability of the water intakes to Seaholm Power Plant and the Thomas C. Green

Water Treatment Plant by replacing a rock weir located upstream of South First Street. The original

design of the dam included plans for a dam without the highway bridge as well as with the highway

bridge. The original study found that a highway bridge could be constructed across the river as part of

the dam for much less than the cost of a separate bridge (Brown & Root 1958). There are no

hydroelectric generating facilities at the dam, although there have been several studies regarding

hydroelectricity at Longhorn Dam, the most recent in 2011 (Toohey 2011).

The dam has seven, 50‐foot wide by 13‐foot tall manually operated lift gates and two, 50‐foot wide by 9‐

foot tall automated bascule gates. The spillway gates discharge into an 85‐foot long concrete bottomed

stilling basin. The stilling basin has an end sill that maintains 7 feet of water behind the gates to dissipate

energy. Figure 2 shows a cross section of the dam and stilling basin.


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Figure 2: Cross Section of Dam and Stilling Basin through Lift Gate

2.2 GATEOPERATION

The two bascule gates were designed to operate automatically and maintain the lake near elevation

428.25 ft‐msl without intervention from operating personnel. Unfortunately, the gates are no longer

able to perform this function well (Freese and Nichols 2010). The bascule gate operators are an

outdated mechanical and hydraulic system that requires frequent adjustments. The controls consist of

an elaborate system of valves, cables and pulleys. Adjusting the gate requires trial and error movements

of the cables and valves. The moving parts are worn and difficult to stabilize, causing the gate to require

additional adjusting. When this occurs, personnel must visit the dam several times a day to adjust the

gates. With the Holly Power Plant closing, personnel will be located further from the dam and frequent

visits to the dam will become more difficult with corresponding time delay to the gate operations.


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Figure 3: Downstream side of automated bascule gate.

When the river flow exceeds the bascule gates’ capacity, personnel must visit the dam and open the lift

gates. The need to open these gates frequently occurs after normal working hours and with limited

warning. The lift gates have a long history of poor operation, failing to open or close properly. Currently,

the guide wheels on Gate 1 have broken off the side of the gate, and the gate is jammed in the slot,

rendering the gate inoperable. The lift gates tend to open unevenly and rack in the gate guides.

Fortunately, each time a gate has been racked; Austin Energy has been able close the gate before the

lake level dropped significantly. Lifting cable failures are also common, making gate operations

impossible until the cable is repaired. The lift gates are opened with an electric hoist that lifts from one

side of the gate which contributes to the gate operating difficulties.


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Figure 4: Downstream view of lift gate 3.

Determining when and how much to open the lift gates requires trial and error. The amount gates are

typically opened is based on the experience the Holly Power Plant personnel have gained over the years.

Once a gate is opened, the lake level is monitored and if it begins to rise or fall more than desired,

additional adjustments are made.

The lift gates are operated with controls located at each pier. In order to access the controls, personnel

must open the door on the pier. The open door blocks the sidewalk, which is also the Hike and Bike Trail.

The roadway is a few feet from the pier doors. A concrete barrier and chain‐link fence have been

installed to protect personnel from motorists.


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Figure 5: Gate operation occurs on this sidewalk shared with the Hike and Bike Trail

2.3 GATEDSPILLWAYRATINGCURVE

The spillway was designed to maintain the full capacity of the river channel without materially increasing

the water level upstream over that which would prevail prior to the dam construction. The nine gates

have the discharge capacity to maintain a constant lake level up to approximately 42,000 cfs. At this

point, the tailwater below the dam is approximately equal to the lake level and the lake level is

controlled by the river level more than the spillway gates. The automated bascule gates were designed

to allow the dam to pass more than 7,500 cfs with the lake at the normal pool elevation of 428.25 ft‐msl

without opening a lift gate. Once this flow rate is exceeded, the manual lift gates must be opened to

prevent the lake from rising. The original designers assumed that due to the river control by Tom Miller

Dam and Mansfield Dam there would be adequate time provided to raise the lift gates for any flood

(Brown & Root 1958).


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Figure 6 shows the rating curve assumed for the dam. This rating curve is used to model water surface

elevations through the City due to flood events. The rating curve assumes that the lake level at

Longhorn Dam does not rise until flows exceed the gate capacity. This rating curve is the best case

scenario assuming perfect gate operation and, as discussed in the following section, is unlikely to occur

during storm events.

Figure 6: Gated Spillway Rating Curve – Best Case Scenario

2.4 HISTORICALPERFORMANCEOFEXISTINGDAM

The gated spillway rating curve shown in Figure 6 is based on the best case scenario for gate operation.

The rating curve is presented based on the assumption that all of the gates are fully functional and

opened just enough to anticipate increased flows but not too much to waste water by lowering the lake

level. Operating the gates in this manner is challenging. The original specifications for the automated

bascule gates allowed for the lake to fluctuate within plus or minus 3 inches of normal pool and allowed

the lake to rise six inches before the bascule gates are fully opened (Brown & Root 1959). Figure 7 shows

typical fluctuations of the lake level with LCRA generating through two turbines. The figure shows that

Gate rating curve assumes all gates are functional and opened in a timely manner.


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the bascule gate generally causes the lake to fluctuate one foot per day and that the lake is frequently

above 429 ft‐msl. Replacing the bascule gate controls with a modern system will improve this

performance but will still allow the lake to fluctuate approximately six inches.

Figure 7: Historical Normal Levels with Releases from Two Turbines at Tom Miller Dam

The manual lift gates would not typically be opened until the river flows exceed the bascule gate

capacity. As seen in Figure 7, the lift gates would generally not be opened until after the lake reaches

429 ft‐msl and if the flow rates continued to increase. Due to the flash flood nature of the Colorado

River and the limited storage volume in Lady Bird Lake, determining when to open lift gates is

challenging. LCRA shares river flow predictions with Austin Energy and notifies Austin Energy of gate

operations at Tom Miller Dam. This coordination with LCRA helps with the gate operations but there is

still uncertainty to flood timing. The river flow rate can increase in a few hours from a volume easily

handled by the bascule gates to a volume requiring five lift gates to be open. Additionally, lake levels

often rise during non‐working hours, extending the response time for a gate operator. Now that

personnel are no longer stationed at Holly Power Plant 24‐hours a day, the response time may increase

even more.


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Historical lake level records from November 13, 2001 through December 13, 2012 were reviewed In

order to determine how the dam has performed in the past. In Figure 8, each dot represents one hour

during the time period mentioned above, and each dot’s position indicates the river flow rate and lake

level elevation during that hour. The figure shows that the lake elevation has frequently exceeded the

elevation predicted by the rating curve for the gates.

Figure 8: Historical Lady Bird Lake Levels compared to Best Case Gate Operation

2.5 GATEREHABILITATIONPROJECTSUMMARY

Longhorn Dam is in need of numerous repairs and upgrades to improve the reliability of the dam and

spillway gates. The existing condition of the dam is documented in a 2010 inspection report (Freese and

Nichols 2010). Following the inspection, FNI prepared a conceptual design to improve gate operations

and reliability and provide enhancements to the downstream channel, river banks and the Hike and Bike

Trail. The opinion of probable construction costs for the conceptual design was $14.2 million (Freese and

Nichols, April 2012). After reviewing the conceptual design, Austin Energy prioritized the repairs and


13

determined that $10 million in repairs were dam operation and maintenance priorities. The key repairs

prioritized by Austin Energy are summarized in Table 3 along with the benefits of these features.

Table 3: Key Features of Gate Rehabilitation Conceptual Design

Repair Benefit

Replace bascule gate controls Bascule gate will maintain normal lake level in tighter range than with existing system

Replace lift gate hoists Lift gates will open and close evenly, reducing likelihood of gates becoming stuck or the cables coming loose.

Construct hoist platform on piers Hoist platform will move operations away from the public sidewalk reducing Hike and Bike Trail closures.

Recoat bascule and lift gates Gates will be protected from corrosion for many years.

Provide second set of stop logs Duration of construction will be reduced.

Provide remote monitoring. Allow remote observation of the dam to verify performance and security.

Replace electrical system Provides second power feed to operate gates in case of power failure. Replaces obsolete electrical parts.

The conceptual design for the gate improvements replaced the controls for the automated bascule gate

with modern electronic controls. The new controls would be expected to reduce the daily fluctuations in

lake level to approximately six inches. The conceptual design for the lift gates replaced the lift gate

hoists that contribute to the gate racking and becoming stuck. The lift gates would remain manually

operated, requiring personnel to respond to floods and determine when to open the lift gates. While

not included in the conceptual design, the manual controls could be located at a remote location, which

would eliminate the travel time needed to reach the dam and open the gates. It would not, however,

reduce the amount of time required to recognize that the gates should be opened.


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3.0 FIXEDWEIR

3.1 FIXEDWEIROVERVIEW

A fixed weir is an overflow structure with no moving parts. It can consist of a concrete wall, a rounded

spillway, or a high spot in a channel. Since there are no moving parts, it is “fixed” in position, as opposed

to a gated structure. Fixed weirs have several advantages to gated structures. They do not require

operation during storm events eliminating human and mechanical error and their maintenance

requirements are typically less.

The chief advantage of gated structures is discharge capacity. Discharge from a dam is related to flow

area. Gated structures increase flow area by opening gates, thus increasing the area below the gate to

pass flow. Gated structures can maintain a nearly constant lake level for a wide range of discharges.

Because fixed weirs do not have moving parts, the only way to increase the flow area at a given weir,

and thus the discharge, is for the lake to rise. For a given channel width, a gated structure will typically

have a higher discharge capacity than a straight fixed weir.

Labyrinth weirs have been developed in order to obtain the benefits of fixed weirs without sacrificing

the discharge capacity of gated structures. Labyrinth weirs are linear weirs that appear folded in plan‐

view to increase the crest length for a given width. A plan view of a typical labyrinth weir section is

shown in Figure 9. A labyrinth weir is able to pass large discharges at relatively low heads compared to

traditional weirs of equal width. Each triangle shape of the labyrinth weir is called a cycle.

Figure 9: Plan View of Typical Labyrinth Weir


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3.2 LABYRINTHWEIREXAMPLES

As a result of their hydraulic performance and geometric versatility, labyrinth weirs have been placed in

streams, canals, rivers, ponds and reservoirs as headwater control structures, energy dissipaters, flow

aerators, and spillways. Labyrinth weirs are well suited for spillway rehabilitation where aging

infrastructure, dam safety concerns, freeboard limitations, and a revised and larger probable maximum

flow have required increased spillway capacity. Labyrinth weirs have been constructed since as early as

the 1950s. Many of them were constructed at existing dams as a replacement to gated spillways. FNI

designed Lake Brazos Dam labyrinth weir, shown in Figure 10 as a replacement to a gated spillway on

the Brazos River near Waco, Texas. The innovative labyrinth weir design combined reuse of the existing

dam site with an unconventional spillway configuration and yielded substantial reduction in

construction cost and time. Following construction, the project received numerous awards for its

innovative design approach and successful construction (Vasquez et al. 2007).

Figure 10: Lake Brazos Dam shortly after construction

Lake Townsend Dam impounds the primary water supply for the City of Greensboro, North Carolina

(Figure 11). A 300‐foot wide labyrinth weir was chosen to replace the existing, deteriorating concrete

gated spillway to provide additional spillway capacity and a low maintenance structure for the City

(Schnabel 2013). This project was also an award‐winning rehabilitation project.


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Figure 11: Lake Townsend Labyrinth Weir under Construction (Schnabel, 2013)

The San Antonio River Authority constructed a labyrinth weir on Elmendorf Lake in 1995 to replace a

gated structure built in 1973. The 12‐foot tall weir is 350 feet wide and has 1,700 linear feet of weir

(Figure 12).

Figure 12: Elmendorf Dam


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3.3 PROPOSEDLABYRINTHCONFIGURATIONATLONGHORNDAM

A labyrinth weir could be constructed at Longhorn Dam on the existing stilling basin. A rendering of the

labyrinth weir is shown in Figure 13. The labyrinth weir consists of approximately 17 cycles and 2,200

linear feet of wall. Two different configurations have been developed, each with advantages. The first

configuration is shown in Figure 14. The cycles are connected to the existing bridge piers. The second

configuration is shown in Figure 15. The cycles do not connect with the existing bridge piers. The

configuration that connects to the piers may simplify construction by making it easier to dewater the

work areas. The other configuration has approximately 100 more feet of weir length and avoids

modifying each pier to connect with the labyrinth weir. The benefits of each configuration should be

evaluated if design of the labyrinth weir continues. A 10‐foot wide by 10‐foot tall slide gate has been

included in both configurations. The slide gate is not needed to pass flood flows but can be used to

lower the lake for hydrilla control or other reasons.

The current labyrinth concept removes all nine gates and provides a sluice gate to lower the lake for

maintenance. Another option would be to replace eight of the gates and leave one of the existing gates

to lower the lake for maintenance. The remaining gate would require repairs to improve reliability. If all

nine gates are removed, the gate piers which currently support the mechanical equipment would be

available for other uses, such as widening the Hike and Bike Trail.

Figure 13: Rendering of Labyrinth Weir at Longhorn Dam


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Figure 14: Labyrinth Weir at Longhorn Dam with cycles connected to existing bridge piers

Figure 15: Labyrinth Weir at Longhorn Dam with cycles not connected to existing bridge piers


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3.4 LABYRINTHWEIRRATINGCURVE

The labyrinth weir was sized to maintain the existing rating curve as much as possible. The crest of the

labyrinth weir was set at elevation 427.6 ft‐msl, approximately 8 inches below the current normal pool

level of 428.25 ft‐msl. This crest level was set so that the lake would be 428.25 ft‐msl when two turbines

are discharging at Tom Miller Dam. The labyrinth rating curve is shown in Figure 16 along with the best

case scenario gate rating curve. Table 4 shows flow rates, lake levels and the difference between the

gated spillway lake elevations and the labyrinth weir lake elevations for the various storm events. Lake

elevations for events up to and slightly beyond the 25‐year event would be somewhat higher with the

labyrinth weir than with the gated spillway. The lake levels for the gated and labyrinth spillways would

be nearly equal approximately midway between the 25‐year and 50‐year storm events. Thereafter, the

reservoir elevations would be the same for both the gated spillway and the labyrinth. The rating curves

shown in the following figures for the existing structure assume that all gates are functional and opened

in a timely manner to manage flows. The labyrinth rating curve assumes that it is free from debris which

would reduce the weir efficiency and cause increased lake levels.

Figure 16: Rating Curve for Gated Structure and Labyrinth Weir

Gate rating curve assumes all gates are functional and opened in a timely manner. Labyrinth rating curve assumes the weir is free from debris and sediment.


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Table 4: Lake Levels at Various Storm Events

Event Discharge

(cfs)

Gated Spillway

Headwater

(ft‐msl)

Labyrinth Weir

Headwater (ft‐msl)

Increased Headwater Due to Labyrinth

Weir

(ft)

2‐year 14,564 428.3 429.3 1.0

5‐year 24,567 428.3 430.0 1.7

10‐year 29,945 428.3 430.3 2.0

25‐year 50,050 430.7 431.7 1.0

50‐year 90,046 439.2 439.2 0.0

100‐year 90,361 439.2 439.2 0.0

Figure 17 shows the historical lake level compared to the labyrinth weir rating curve. The figure shows

that the lake level has frequently exceeded the elevation predicted by the labyrinth spillway rating

curve. While the true flood performance of the existing structure must include inefficiencies in gate

operation due to maintenance and operational limitations, the labyrinth spillway would be expected to

more closely follow the predicted rating curve because the labyrinth passively responds during flood

events.

Figure 17: Historical Lady Bird Lake Levels vs. Predicted Rating Curves


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There are design modifications that could be made to the labyrinth to mitigate the head rise. The crest

could be lowered even more, but that would also lower the normal pool level. The angle of the labyrinth

cycles could be reduced, which would increase the length of each cycle and the overall weir length. A

third option would be a fractal labyrinth weir. A fractal labyrinth weir would consist of constructing a

small labyrinth alignment out of steel plate to act as the overflow crest along the top of the large

labyrinth walls (Figure 18). The fractal labyrinth could be constructed on the entire large labyrinth or

only on a few of the cycles. Fractal labyrinths have not been implemented in major civil projects, but a

similar concept has been laboratory tested with promising results for lower heads (Erpicum, et al. 2011).

The fractal labyrinth could potentially reduce the labyrinth head rise up to the 25‐year event. A

significant modeling effort would be required to evaluate the impact of the fractal labyrinth. The steel

plate could potentially be subject to more damage than the concrete labyrinth, increasing maintenance

obligations.

Figure 18: Fractal Labyrinth Weir

3.5 LABYRINTHPERFORMANCE

In order to evaluate the historical performance of the gated structure and the predicted performance of

the labyrinth weir, FNI prepared visual comparisons of the expected labyrinth performance when it is

subjected to available historical flows. The first visual comparison incorporates more than ten years of

historical data. Figure 19 shows a simulation of what the lake levels would have been with the proposed


22

labyrinth weir, given the historical flows at the dam. The figure also shows the actual lake levels during

this time. Note that given these historical flow rates, the labyrinth would have reduced a number of

peak lake levels and eliminated events where the lake fell below normal pool.

Figure 19: Historical Dam Performance and Predicted Labyrinth Performance

An examination of the data shown in Figure 19 for significant flood events and typical operation is

shown in Figure 20 through Figure 23. Figure 20 shows that the highest lake level during the November

2004 flood would have been reduced by 2.8 feet with the labyrinth in place. The figure also shows that

between November 26 and December 2, the lake level with the labyrinth would have been

approximately 1.5 feet higher than what actually occurred with the gated spillway. The performance of

the labyrinth was also compared to the Tropical Storm Hermine flood of September 2010 and the

labyrinth would have reduced the peak lake elevation for that storm event as well (Figure 21).

The performance of the labyrinth was also compared to the historical performance when LCRA is

generating power at Tom Miller Dam. Figure 22 and Figure 23 show the dam performance for one and

two turbine turbines releasing at Tom Miller Dam. There has historically been a 6 to 15 inch fluctuation

in lake levels between generation releases and no generation releases. The fluctuation with the

labyrinth will be similar.


23

Figure 20: Observed and Predicted Lake Levels for November 2004 Flood

Figure 21: Observed and Predicted Lake Levels for September 2010 Flood (Tropical Storm Hermine)

Peak lake level would not have occurred with labyrinth in place.

Higher lake level would have occurred with labyrinth in place.

Peak lake level would not have occurred with labyrinth in place.


24

Figure 22: Observed and Predicted Lake Levels during typical LCRA Releases from One Turbine

Figure 23: Observed and Predicted Lake Levels during typical LCRA Releases from Two Turbines


25

3.6 BACKWATERANALYSIS

Section 3.4 showed that the labyrinth weir increases the lake level at Longhorn Dam by up to 2 feet

compared to the best case gate operation. Section 3.5 showed that the labyrinth weir will reduce peak

lake levels during storm events by eliminating the need to operate gates. Section 3.5 also showed that

once there has been enough time to properly adjust the gates, the gated structure will maintain lower

lake levels than the labyrinth weir. These analyses were based on the lake level at Longhorn Dam. In

order to determine what impact the elevated lake levels at the dam may have upstream, the City’s HEC‐

RAS effective models were used. These models assume that the lake level remains constant at Longhorn

Dam until approximately 40,000 cfs. As mentioned previously, this is not typically the case.

Flows for the 2‐year event to the 100‐year event were modeled, along with steady flow rates of 25,000

and 30,000 cfs representing the large historical floods shown in Table 2 that had extended periods of

relatively steady high flows. Results of the modeling are shown in Appendix A. Observations from the

backwater analysis will be discussed in later sections.


26

4.0 LABYRINTHWEIRASSESSMENT

4.1 LAKELEVELS

The labyrinth weir will result in less fluctuation in lake levels than the gated structure has had

historically. As seen in Figure 8 the lake has dropped below normal pool many times due to improper

gate operation. This represents lost water that LCRA must release to refill Lady Bird Lake. The labyrinth

weir will reduce peak lake levels during storm events but will cause a longer duration of higher lake

levels.

4.2 HIKEANDBIKETRAIL

The Hike and Bike Trail and the new Boardwalk are located on Lady Bird Lake. Austin Energy surveyed

many locations along the river to obtain elevations for the Hike and Bike Trail. The survey points are

included in Appendix D. The City’s Public Works Department provided a copy of the Boardwalk drawings

which included the elevations of the Boardwalk. The backwater analysis showed that with best case gate

operations the Boardwalk will be above the 25‐year storm event lake level and submerged by the 50‐

year event. A small section of the Boardwalk will be submerged by the 25‐year storm event with the

labyrinth weir. Results from the backwater analysis are shown in Appendix A.

For the labyrinth weir, low spots on the existing Hike and Bike Trail will be inundated during the 5‐year

event, while under best case gate operation conditions these low spots would be inundated during the

10‐year event. Due to the uncertainty with current gate operation, the Hike and Bike Trail may be

inundated earlier than the 10‐year event.

4.3 CESARCHAVEZSTREETATLAMARBOULEVARD

There is a low spot on West Cesar Chavez Street beneath the Lamar Boulevard Bridge. This area has

historically flooded and caused Cesar Chavez to be closed. FNI searched for records indicating when and

for how long the roadway has been closed but did not find any data. Discussions with the City of Austin

Watershed Department (Watershed) indicated that the flooding may be due to the lake backing water

into the culvert in this location. Reportedly, Watershed now blocks the culvert during large floods to

prevent lake water from flowing from the lake into the street and then pumps local drainage from the

roadway.


27

The backwater analysis showed that Cesar Chavez may flood beginning in the 5‐year event with the

labyrinth weir and in the 10‐year event with the gated structure. The analysis with the steady flow rates

representing extended floodwater releases from Mansfield Dam indicated that Cesar Chavez would not

flood with the existing structure as long as the gates are operated correctly but may flood with the

labyrinth. There may be engineering methods available to prevent the flooding of Cesar Chavez, such as

the installation of a backflow preventer and sump pump in the culvert. The backwater analysis also

indicated that constrictions in the river between South First Street and South Lamar Boulevard increase

the water level. These constrictions include the bridge piers and possibly remnants of an old rock dam

and sediment that accumulated behind the rock dam. Removal of the rock dam remnants and sediment

may reduce the likelihood of flooding at West Cesar Chavez Street and Lamar Boulevard.

4.4 EXISTINGSTRUCTURESANDTHE100‐YEARFLOODPLAIN

The labyrinth weir and gated structure will perform similarly for the 50‐year and 100‐year storm events.

The labyrinth will not increase the lake levels at these events. Watershed provided finished floor

elevations of structures along the river. The lowest structure is Joe’s Crab Shack on Riverside Drive. The

backwater analysis indicated that Joe’s Crab Shack would not be inundated by either the gated structure

or the labyrinth weir at the 25‐year event but would be inundated during the 50‐year event with either

structure.

4.5 DEBRISANDSEDIMENTATION

While some debris washes over labyrinth weirs, large debris such as trees can become caught in the

cycles. Debris buildup can reduce the labyrinth weir discharge capacity. Debris will need to be cleaned

from the labyrinth weir. A debris boom could be installed upstream of the dam to collect debris and

prevent it from accumulating on the labyrinth weir. An access road could be provided downstream of

the labyrinth to facilitate debris removal from the downstream side. The renderings shown in Figure 13

through Figure 15 show a debris boom upstream of the dam. Figure 13 also includes an access road on

the downstream side.

Watershed Field Operations Division currently cleans debris from the lake and removes more than 200

tons of trash from the lake annually. There are already several debris booms in the lake that capture

debris upstream and reduce the amount reaching Longhorn Dam. While removing debris from the


28

debris boom or from the labyrinth would increase the lake cleaning effort, it would also have the

environmental benefit of removing the trash from the lake and not allowing it to pass downstream.

Sedimentation can reduce labyrinth weir discharge if significant sediment accumulates in the cycles.

Lady Bird Lake has not experienced significant sedimentation. No issues with sediment accumulation are

anticipated but the possibility of sedimentation should be evaluated.


29

5.0 STAKEHOLDERMEETINGS

Lady Bird Lake is a valuable asset to the downtown area. Hence, there are many parties who have an

interest in the lake. Austin Energy and FNI held meetings with many of the stakeholders to identify

concerns regarding the labyrinth weir.

5.1 PUBLICWORKS

A meeting was held with Pirouz Moin, Supervising Engineer, Infrastructure Management Group of the

Public Works Department on March 22, 2013 to discuss the labyrinth weir’s impact on the Pleasant

Valley Bridge. The two labyrinth configurations were presented. Mr. Moin preferred the option that did

not connect the weir to the existing piers. He highlighted the need to consider differential movement

between the existing structure and the labyrinth weir. Changes in water surface elevation during storm

events for both structures were discussed. Mr. Moin did not see any major issues with the labyrinth

related to his group. At the time of the meeting, the backwater analysis had not been performed and

the potential flooding at Cesar Chavez Street and Lamar Boulevard had not been identified and was not

discussed in the meeting.

Mr. Moin also asked about the impact the changed water levels may have on the Boardwalk that is

currently under construction. He put us in touch with David Taylor, Boardwalk Project Manager, who

provided copies of the Boardwalk drawings. Mr. Walker asked if the mechanical equipment would still

be needed at the dam if the labyrinth weir were built. During planning on the Boardwalk, public works

looked into using the dam to widen the Hike and Bike Trail. He expressed interest in using the dam for

the Hike and Bike Trail if it would not interfere with the dam mechanical equipment.

5.2 PARKSANDRECREATION

A meeting was held with Charles Vaclavik and Ricardo Solis, division managers of the Parks and

Recreation Department (PARD) along with several other PARD personnel on March 29, 2013. The

labyrinth weir concept was presented and discussed. The historical performance of the existing dam

was discussed including the quick lake rise during a flood due to timing of gate operations. Similarly, the

performance of the labyrinth weir was discussed including the reduction in peak water surface elevation

and longer duration of elevated lake levels for certain events. The potential for a lower normal pool if a

labyrinth weir was built was also discussed. PARD indicated that vendors on the lake may be impacted


30

by a lowered normal pool level. The vendors have floating docks and the level change may affect the

dock access.

PARD was concerned about the impact to the Hike and Bike Trail during construction at the dam. FNI

explained that modifications to the gates will require significant work from the bridge while the

labyrinth construction would likely have less impact since most of the labyrinth construction will take

place below the bridge. We also discussed that if the labyrinth is constructed and the existing gate hoist

equipment is removed, there is a possibility that the piers could be used to improve the Hike and Bike

Trail over the dam in the future.

Following the meeting, PARD identified a few areas along the Hike and Bike Trail that have flooded in

the past. These areas were at Auditorium Shores between the Stevie Ray Vaughn Statue and the Pavilion

on the south side of the river and the entire trail from Waller Creek to Festival Beach on the north side

of the river as well as the peninsula at Holly Power Plant. Maps of the flood prone areas provided by

PARD are included in Appendix B. The backwater analysis in Appendix A shows the storm events that

impact these areas.

5.3 WATERSHEDPROTECTIONDEPARTMENT

Several meetings were held with Watershed to discuss the labyrinth weir. Kevin Shunk, (Floodplain

Management, Flood Early Warning System), provided finished floor elevations for several structures

along the river. Once the backwater modeling was complete, FNI met with Mr. Shunk and Eduardo

Acosta to discuss the results. The drainage criteria manual requires stormwater runoff peak flow rates

for the two, ten, twenty‐five and one‐hundred year frequency storms to not cause increased inundation

of any building or roadway surface. There was discussion that the drainage criteria manual was

developed mainly for uncontrolled streams. Permitting the labyrinth weir will require some discussion

within Watershed to determine how to apply the drainage criteria manual for the project. The baseline

water surface in the model is based on the best case scenario gate operations. Since the best case

scenario is not representative of the historical operations, a different baseline may be needed for this

project. Mr. Shunk indicated that Watershed’s Field Operations Division cleans the lake. No coordination

has taken place with this division to discuss the potential debris on the labyrinth.


31

5.4 AUSTINWATERUTILITY

FNI evaluated Austin Water Utility (AWU) as a stakeholder to the project. AWU has water and

wastewater lines that cross beneath the lake as well as manholes located on the shore. Since the

difference in water surface elevations is only two feet at small events and there is no difference for the

50‐year and larger events and the lake has historically exceeded the predicted lake levels with the

labyrinth weir, no impact to the Utility is anticipated. Further consultation in the future may be required.

5.5 LOWERCOLORADORIVERAUTHORITY

A meeting was held with LCRA regarding the labyrinth on April 8, 2013. FNI and Austin Energy

introduced the concept of a labyrinth weir for Longhorn Dam. LCRA was enthusiastic about the project.

LCRA is required to maintain minimum flow rates at downstream river gages. The bascules gates can be

unpredictable and complicate LCRA’s river operations. When the bascule gates cause the lake to drop

below normal pool, LCRA must release additional water to fill the lake before the bascule gates release

the flow. As a passive structure, the labyrinth weir will be more predictable than the bascule gates and

will not allow the lake to drop below normal pool.

5.6 LAKEBRAZOSDAMSITEVISIT

Representatives from Austin Energy met with Ricky Garrett, City of Waco Director of Utility Services on

April 17, 2013 to discuss their experience with the labyrinth weir at Lake Brazos Dam. Mr. Garrett

discussed the history of Lake Brazos Dam and the reasons for converting the gated spillway to a

labyrinth weir. The gates were unreliable and frequently failed rendering the lake non‐existent for

extended periods. The City of Waco has been happy with the labyrinth weir performance. Debris collects

on the weir and has been removed five times in six years. The dam was scheduled to be cleaned in May

2013. Following the visit with Mr. Garrett, the group visited Lake Brazos Dam to observe the weir and

debris.


32

6.0 ENVIRONMENTALANDPERMITTINGCONSIDERATIONS

6.1 ENVIRONMENTALPERMITTING

An environmental site assessment was performed as part of the conceptual design for the gate repair.

The assessment searched for items that could impact the project or require additional permitting. No

sites such as former underground storage tanks and other facilities that may have contaminated the soil

were found that have the potential to have caused conditions of environmental contamination within

the project limits. No mapped historical or cultural places were found within the project limits, therefore

no impacts are expected to historical or cultural places due to the project construction. No critical

environmental features or threatened or endangered species were located in the project area. No

impacts to the project are anticipated based on the environmental study. It is anticipated that the

proposed labyrinth modifications can take place under U.S. Army Corp of Engineers Section 404

Nationwide Permit which allows maintenance to existing dams.

The City of Austin tree protection ordinance will apply to this project. Trees larger than 8 inches in

diameter are protected and may not be removed unless the Watershed Protection and Development

Review Department (WPDRD) has issued a permit for the removal. There are a few protected trees in

the project area. Coordination with WPDRD will occur as part of final design. WPDRD may require

mitigation, including the planting of replacement trees, as a condition of application approval.

The project is located on City parkland. Coordination with PARD will occur to coordinate easements for

work areas, material storage sites and access roads.

6.2 TCEQDAMSAFETY

The Dam Safety Program of the Texas Commission on Environmental Quality (TCEQ) monitors and

regulates dams in the State of Texas. Texas Administrative Code requires that dam owners submit final

construction plans and specifications to TCEQ for review and approval before commencing construction

or repairs to an existing dam. The final design will be subject to TCEQ approval and will be in accordance

with TCEQ criteria.

6.3 STATEHISTORICPRESERVATIONOFFICE

The Antiquities Code of Texas was passed in 1969 and requires that the Texas Historical Commission

staff review any action that has the potential to disturb historic and archeological sites on public land.


33

Actions that need review under the Antiquities Code of Texas include any construction program that

takes place on land owned or controlled by a state agency or a state political subdivision, such as a city

or a county. Projects that require review include:

Reservoirs constructed by river authorities and water districts;

Construction of recreational parks or the expansion of existing facilitates by city governments;

Energy exploration by private companies on public land; and

Construction by a city or county government that exceeds 5 acres or 5,000 cubic yards,

whichever comes first. If the activity occurs inside a designated historic district or affects a

recorded archeological site, it needs to be reviewed, regardless of project size.

Rehabilitation or demolition of a building owned by a state agency or university that is listed or

eligible to be listed in the National Register of Historic Places (NRHP).

The State Historic Preservation Office (SHPO) was consulted in the conceptual design for the gate repair

project. The dam is not listed as a State Archeological Landmark (SAL) or in NRHP. However, the dam

may be eligible for listing in the National Register of Historic Places (NRHP). If the project does not

require a permit from the Corp of Engineers, a review by SHPO will not be required. However, a need for

a Corp of Engineers permit is anticipated, therefore an evaluation of NRHP eligibility would need to be

performed and a thorough evaluation of the project would need to be made by SHPO during final

design. The project will likely require a permit from the Texas Historical Commission.


34

7.0 OPINIONOFPROBABLECONSTRUCTIONANDOPERATIONCOSTS

7.1 GATEREHABILITATIONPROJECT

The gated structure has served the City for over 50 years and is in need of repairs. As discussed in

Section 2.5, after reviewing the gate rehabilitation conceptual design, Austin Energy prioritized the

repairs and determined that $10 million in repairs were dam operation and maintenance priorities.

Once the dam has been repaired, there will be ongoing operation and maintenance (O&M) costs. Austin

Energy reports that the gates have historically been recoated every ten years. This frequency is unusual

as 25 to 30 years is more typical for spillway gates. A 15‐year frequency for gate coatings was selected

for further cost analysis. The remote monitoring and control system for the gates will also need to be

updated every 15 years. Operation of the gates will require that trained personnel be available 24 hours

a day to operate the lift gates in storm events and verify proper operation of the automated bascule

gates regularly. Austin Energy estimated the need for six trained personnel assigned halftime to the dam

and provided labor costs for these personnel. The estimated costs to repair, operate and maintain the

gated structure are shown in Table 5.

Table 5: Gated Structure Repair, Operation and Maintenance Item Costs

Item Frequency Cost

Engineering Design One Time $600,000

Rehabilitation Construction One Time $10,000,000

Gate Coatings 15‐years $1,500,000

Control System Upgrades 15‐years $100,000

O&M Labor Costs Annual $360,000

O&M Expense Costs Annual $125,000

7.2 LABYRINTHWEIRPROJECT

Design of the labyrinth weir will require the use of computer and physical models to further evaluate the

labyrinth performance and refine the labyrinth discharge rating curve. The study will also verify the

gated structure rating curve. These studies will increase the duration of the design over the gate

rehabilitation project. The opinion of probable construction cost for the labyrinth weir is $15.0 million;

details of the probable cost are included in Appendix D. The cost includes a service road to access the

downstream side of the labyrinth and to drive behind the entire width of the labyrinth. This road can be


35

used to facilitate debris removal. As an alternate, this access road could be eliminated and debris

removed with a crane on Pleasant Valley Bridge but would impact traffic. The elimination of the access

road would reduce the probable construction by approximately $1 million. Schedules for the two

alternatives are included in Appendix F. The labyrinth weir will have small annual operational cost. The

slide gate should be exercised annually to verify that it functions. Debris will need to be removed on an

as‐needed basis. The estimated costs to construct and operate the labyrinth weir are shown in Table 6.

Table 6: Labyrinth Weir Construction and Operation Costs

Item Frequency Cost

Engineering Design One Time $1,500,000

Initial Cost One Time $ 15,000,000

Debris Cleaning Annual $20,000

Exercise Gate and Miscellaneous Observation Annual $5,000

7.3 LIFECYCLECOSTS

The present value and the equivalent annual worth of the gated structure and the labyrinth weir were

calculated for a 50‐year life cycle and 4% interest rate and are shown in Table 7. Based on the assumed

operation and maintenance costs, the labyrinth weir is less expensive than the gated structure.

Table 7: Life Cycle Costs

Gated Structure Labyrinth Weir

Present Value $20,000,000 $17,040,000 Equivalent Annual Worth $930,600 $793,100


36

8.0 CONCLUSION

8.1 COMPARISON

Longhorn Dam is in need of improvements. Both the gate rehabilitation and the labyrinth weir provide

many benefits over the existing condition. Table 8 summarizes the attributes for each of the two

configurations of Longhorn Dam as discussed in this preliminary investigation.

Table 8: Attributes of Alternatives

Factors Alternative

Gate Improvements Labyrinth Weir

Initial Engineering and Construction Cost

$10,600,000 $16,500,000

50‐year Present Value $20,000,000 $17,040,000

Flood Operation Requires personnel at any time of day to operate gates

No operation required

Debris Removal None Approximately once per year

Peak Lake Levels Lake rises quickly during storm events due to delay in gate operation

Instant and passive flood response significantly reduces peak lake levels for storm events

Lake levels during extended flood events

After period of gate adjustment, lake level at dam can be near normal pool

The lake level will remain elevated during extended flood events or flood releases from upstream dams

Lake level reliability Lake level is a function of inflow and gate opening configuration, which leads to significant variation

Lake level is only a function of inflow leading to consistent and predictable lake levels

Wasted Water Gates can allow lake to drop below normal pool

Lake will not drop below normal pool

Maintenance Requires regular maintenance and adjustment

Minimal

Hike and Bike Trail

Removes most operational requirements from the public sidewalk reducing Hike and Bike Trail closures

May allow the piers to be used to support the Hike and Bike Trail


37

8.2 CONCLUSION

This report presented a conceptual design for a labyrinth weir at Longhorn Dam, evaluated the hydraulic

feasibility and identified stakeholders to the project. The analysis found that the labyrinth weir would

improve the operational requirements of Longhorn Dam by eliminating the need for gate operations.

Peak lake elevations during flood events would likely be reduced by eliminating the gate operations. The

lake level would remain elevated during high river flows. Additional analysis and evaluations will be

required to determine if a labyrinth weir is appropriate at Longhorn Dam. Items to consider when

evaluating the labyrinth weir have been discussed previously and include:

Environmental permitting including historic considerations,

Management of river trash and debris,

Reduced ability to lower the level of Lady Bird Lake for upstream maintenance or hydrilla

control,

Impact of increased lake level on Hike and Bike Trail,

Impact of increased lake level on West Cesar Chavez Street

The information presented in this report should be considered conceptual in nature. The replacement of

an existing gated spillway with a labyrinth weir is an involved project requiring detailed analysis and

design.


38

REFERENCES

Brown & Root, Inc., (August 1958). Report on Low‐water Dam: Prepared for City of Austin, Texas.

Brown & Root, Inc., (February 9, 1959). Low Water Dam Contract Drawings.

Brown & Root, Inc., (1959). Low Water Dam Colorado River contract No. 80, Legal Notice, Specification,

Proposal, Contract & Bond Form.

Toohey, Marty. (May 25, 2011). “Company proposes hydropower plant at Longhorn Dam”, Austin

American‐Statesman, accessed May 29, 2013, http://www.statesman.com/news/news/local/company‐

proposes‐hydropower‐plant‐at‐longhorn‐da‐1/nRbLr/

Schnabel (2013). Lake Townsend Dam ‐ Dams and Levees Portfolio. www.schnabel‐eng.com. Accessed

March 4, 2013.

Vasquez, V., Boyd, M., Wolfhope, J., and Garret, R. (2007). “A labyrinth rises in the heart of Texas.”

Proceedings of the 28th Annual USSD Conference. 813‐826.

Halff Associates, Inc., (October 2003). “Flood Damage Evaluation Project: Information Paper: Lower

Colorado River Basin Study ‐ Phase I Problems, Needs and Opportunities,” Report and Appendices A and

B, prepared for Lower Colorado River Authority and U.S. Army Corps of Engineers.

Erpicum, S. and Laugier, F.D.R. and Pirotton, M. and Boillat, J.L. and Reverchon, B. and Schleiss, A.J.

(2011). Development of a new concept of Piano Key Weir spillway to increase low head hydraulic

efficiency: Fractal PKW, Labyrinth and Piano Key Weirs: PKW 2011, CRC Press/Balkema.

Freese and Nichols, Inc. (April 19, 2012). Longhorn Dam Modernization Conceptual Design Report,

Austin, Texas.

Freese and Nichols, Inc. (December 6, 2010). Longhorn Dam Comprehensive Facility Review, Austin,

Texas.


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SL)

DISTANCE UPSTREAM FROM LONGHORN DAM (MI.)

EXISTING DAM

PROPOSED LABYRINTH

POINTS ALONG NORTH TRAIL

POINTS ALONG SOUTH TRAIL

INTERSTATE

35

S. CONGRESS AVE.

S. 1ST ST.

UNION PACIFIC RR

S. LAMAR BLVD

CONFLUEN

CE W/

BARTO

N CREEK

MOPACEX

PY

RED

BUD TRAIL

LONGHORN DAM

TOM M

ILLER DAM

2‐YEAR STORMWATER SURFACE PROFILES

Note: Water surface profile for existing dam assumes instant operation of gates in response to flood flows according to the "best case " rating curve.

A3

420

425

430

435

440

445

450

455

460

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

ELEV

ATION (FT‐M

SL)


EXISTING DAM

PROPOSED LABYRINTH



INTERSTATE

35

S. CONGRESS AVE.

S. 1ST ST.

UNION PACIFIC RR

S. LAMAR BLVD

CONFLUEN

CE W/

BARTO

N CREEK

MOPACEX

PY

RED

BUD TRAIL

LONGHORN DAM

TOM M

ILLER DAM



A4

420

425

430

435

440

445

450

455

460

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

ELEV

ATION (FT‐M

SL)


EXISTING DAM

PROPOSED LABYRINTH



INTERSTATE

35

S. CONGRESS AVE.

S. 1ST ST.

UNION PACIFIC RR

S. LAMAR BLVD

CONFLUEN

CE W/

BARTO

N CREEK

MOPACEX

PY

RED

BUD TRAIL

LONGHORN DAM

TOM M

ILLER DAM



A5

420

425

430

435

440

445

450

455

460

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

ELEV

ATION (FT‐M

SL)


EXISTING DAM

PROPOSED LABYRINTH



INTERSTATE

35

S. CONGRESS AVE.

S. 1ST ST.

UNION PACIFIC RR

S. LAMAR BLVD

CONFLUEN

CE W/

BARTO

N CREEK

MOPACEX

PY

RED

BUD TRAIL

LONGHORN DAM

TOM M

ILLER DAM



A6

420

425

430

435

440

445

450

455

460

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

ELEV

ATION (FT‐M

SL)


EXISTING DAM

PROPOSED LABYRINTH



INTERSTATE

35

S. CONGRESS AVE.

S. 1ST ST.

UNION PACIFIC RR

S. LAMAR BLVD

CONFLUEN

CE W/

BARTO

N CREEK

MOPACEX

PY

RED

BUD TRAIL

LONGHORN DAM

TOM M

ILLER DAM



A7

420

425

430

435

440

445

450

455

460

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

ELEV

ATION (FT‐M

SL)


EXISTING DAM

PROPOSED LABYRINTH



INTERSTATE

35

S. CONGRESS AVE.

S. 1ST ST.

UNION PACIFIC RR

S. LAMAR BLVD

CONFLUEN

CE W/

BARTO

N CREEK

MOPACEX

PY

RED

BUD TRAIL

LONGHORN DAM

TOM M

ILLER DAM

STEADY 25,000 CFSWATER SURFACE PROFILES

Note: Steady flow conditions may develop during extended flood releases from Lake Travis at Mansfield Dam.

A8

420

425

430

435

440

445

450

455

460

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

ELEV

ATION (FT‐M

SL)


EXISTING DAM

PROPOSED LABYRINTH



INTERSTATE

35

S. CONGRESS AVE.

S. 1ST ST.

UNION PACIFIC RR

S. LAMAR BLVD

CONFLUEN

CE W/

BARTO

N CREEK

MOPACEX

PY

RED

BUD TRAIL

LONGHORN DAM

TOM M

ILLER DAM

STEADY 30,000 CFSWATER SURFACE PROFILES

Note: Steady flow conditions may develop during extended flood releases from Lake Travis at Mansfield Dam.


Appendix B Flood Prone Areas Identified by Parks and Recreation

Kealing

SouthAustin

Parker

OldBakeryTreaty

Oak Square

ButlerPark

Lott

Metz

BoggyCreek

BlunnCreek

PeaseHaskellHouse

Lamar Beachat Town Lake

Roy G. GuerreroColorado

River Park

Gillis

Edward RendonSr. Park at

Festival Beach

Zilker

ParqueZaragoza

E 16TH ST

E CESAR CHAVEZ ST

COLO

RADO

ST

RIVER ST

S 1ST

ST

E OLTORF ST

W 5TH ST

W MONROE ST

RABB RD

PARA

MOUNT

AVE

NEW

TON

ST

E 7TH ST

E 17TH ST

E 12TH ST

BOUL

DIN A

VE HOLLY ST

W 9TH ST

W 12TH ST

ROSEWOOD AVE

HILL

SIDE

AVE

E 2ND ST

E 11TH ST

JULIET ST

W 10TH STW 6TH ST

E 3RD ST

W 6TH ST

CLAWSO

N RDW 7TH ST

FORD ST

N IH

35 SV

RD NB

W 5TH ST

W 10TH ST E 18TH ST

E 13TH ST

E 5TH STE 5TH ST

E 8TH ST

E 4TH ST

N IH

35 SV

RD SB

LUPINE LN

E 10 TH ST

W OLTORF ST

BAYL

OR ST

LONG BO W LN

W 17TH ST

COMAL ST

E MONROE STE ANNIE ST

W 2ND ST

S 6TH

ST

BARTON SPRINGS RD

W 15TH ST

S 7TH

STE 6TH ST

S LAMAR BLVD

E 14TH ST

GARN

ER AV

E

E 10TH STE 11 TH ST

W 16TH ST

KENW

OOD

AVE

E 9TH ST

W 9TH ST

S 2N

D ST

S 6TH ST

GUAD

ALUP

E ST

E 8TH ST

STAC

Y LN

SAN

SABA

ST

FOLT

S AV

E

TRAILS IDE DR

WILSO

N ST

KINN

EY AV

E

KINN

EY RD

LAVA

CA ST

E 9TH ST

HETHER ST

ASHBY AVE

E 20TH ST

NEW YORK

AVE

SH 165

E 20TH ST

S 3R

D ST

E 8TH ST

GARD

EN VI

LL

A LN

E 3RD ST E 2ND ST

S 2ND ST

ANTH

ONY S

T

W 11TH ST

E 7TH ST

TRIN

ITY

ST

E 8TH STE 6TH ST

LEONA STE 13TH

STW 10TH ST

E 4TH ST

SALINA STEV

A STBRODIE ST

W 9TH STW 8TH ST

S CON

GRES

S AVE

CONG

RESS

AVE

S 5TH

ST

W MARY ST

W 3RD ST

WATERLOO TRL

S 4TH

ST

SABI

NE ST

S PLE

ASAN

T VAL

LEY R

D

TINN

IN FO

RD RD

E 3RD STHASKELL ST

DOL PHIN

D R

COLU M BUS DR

E 6TH ST

EUCL

ID AV

E

JESS

IE ST

PARKER

LN

RED RIV

ER ST

CORB

IN LN

BU

RTON DR

TOOMEY RD

S LAKESHORE BLVD

N MOPAC EX

PY NB

E RIVERSI DE DR

E LIVE OAK ST

WALL

ER ST

E RIVERS IDE

DRN

LAMA

R BL

VD

S 3RD

ST

DEL CU

RTO

RD B ONH A M

TE R

LO

CKE

LNSTRATFORD DR

S IH

35 SB

N IH

35 SB

N IH

35 SB

LOU NEFF RD

AustinRowing

DockTexasRowingCenter

Town LakeRowingCenter


TownLake RowingCenter

0 600 1,200

Feet ±

This product is for informational purposesand may not have been prepared for orbe suitable for legal, engineering, or

surveying purposes. It does not representan on-the-ground survey and represents onlythe approximate relative location of propertyboundaries. This product has been produced

by the Parks and Recreation Departmentfor the sole purpose of geographic reference.

No warranty is made by the City of Austinregarding specific accuracy or completeness.

LegendLBL Trail Flood Areas

Yes

No

!y Water Access Area

Creek Centerlines

City of Austin Parks

Lady Bird Lake TrailProblem Flood Areas (Plus Red Bud Isle-not shown)

05 April 2013 MS

JohnsonCreek

ButlerShores atTown Lake

BartonCreek

ZilkerNature

Preserve

Veterans

Eilers(DeepEddy)

UmlaufSculptureGarden

Treaty OakSquare

ButlerPark

AuditoriumShores atTown Lake

AustinHigh Tennis

Center

LamarBeach at

Town Lake

Zilker

WestAustin

ShoalBeach at

Town Lake

S LA

MA

R BL

VD

SB

W 5TH ST W 5TH ST

RESERVE RD

W 9TH HALF ST

PRES

SLER

ST

N M

OPA

C EX

PY SB

FOSTER AVE

BAY

LOR

ST

W 10TH ST

W 6TH STW

5TH ST

BAY

LOR

ST

BLAN

CO S

T

W 9TH ST

BARTON SPRINGS RD

JESS

IE S

T

ATLA

NTA

ST

W 9TH ST

N MO

PAC E

XPY N

B

ANDREW

ZILKER RD

HIG

HLA

ND A

VE

W RIVERSIDE DR

WIN

FLO

DR

W 3RD ST

ROSE ST

W 11TH ST

S MOPAC EXPY NB

W 4TH ST

VALE ST

PAU

L ST

S MOPAC EXPY

SVRD SB

FRANCIS AVEW 8TH ST

W 8TH ST

W 10TH ST

POW

ELL

ST

WES

T LY

NN ST

N L

AM

AR

BLV

D

WA

LSH

ST

VANCE LN

HA

RTH

AN S

T

W 2ND ST

BR O WN

LEE

CIR

BARTON B LV

D

STER

ZING

ST

W 7TH ST

S MOPAC EXPY SVRD NB

S LA

MA

R B

LVD

LEE

BART

ON

DR

ZILKER

CLUBHOUSE RD

N LA

MA

R BL

VD

W CESAR CHAVEZ ST

PRES

SLER

ST

ROBE

RT E

LEE RD TOOMEY RD

NA

TURE

CENT

ER DR

STE

PHE

N

F AUSTIN DR

S MOPAC EXPY SB

S LA

MA

R BL

VD

NB

VETERA

N

S DR

BARTON SPRINGS RD

WILLIAM BARTON DR

OA

KLA

ND

AVE

COLUMBUS DR

BARTON

SPRINGS

RD

STRATFORD DR

CESA

R C

HAV

EZ TO

MO

PAC

NB RAM

P

LOU NEFF RD

LOU NEFF RD

428

430

432

434

428

430

432

434

434

432

428434

432

430

432

430

428

430

432

428

428

428

434

434

434

430

430

430

430

428

428

432

432

432

AustinRowingDock

TexasRowingCenter

0 250 500

Feet

Legend

ELEVATION

428

430

432

434

LBL Trail Flood Areas

Yes

No



±Lady Bird Lake Trail Problem Flood Areas-West End

05 April 2013 MS



by the Parks and Recreation Departmentfor the sole purpose of geographic reference.No warranty is made by the City of Austin

regarding specific accuracy or completeness.

Red BudIsle

LAKE CLIFF

CTL

AK

E A

USTIN

BLVD

REDBUD TRL

LAKE C

LIFF

TRL

434

432

432430

43 4

428

428

434

432

428430

432

434

430

430

434

43042

8

434

432

430

428

430

Redbud Isle Inset

0 150 300 450 600Feet

SirSwantePalm

MaryDawson

ButlerShores atTown Lake

DoughertyArts Center

WallerCreek

NicholasDawson

EastBouldinCreek

ButlerPark

AuditoriumShores atTown Lake

NorwoodTract at

Town Lake

Sanchez

BlunnCreek

WestBouldinCreek

WallerBeach at

Town Lake

Shoal Beachat Town

Lake


Festival Beach

N IH

35

SVR

D S

B

BARTON SPRINGS RD

N IH

35

NB

NASH HERNANDEZ SR RD

N IH

35 SV

RD

NB

E CESAR CHAVEZ ST

BOU

LDIN

AVE

ALT

A V

ISTA

AVE

ALAMEDA

DR

TRIN

ITY

ST

W ELIZABETH ST

CO

NG

RES

S AV

E

HILLSI

DE

AVE

RED

RIV

ER ST

ACADEMY DR

POST OAK ST

N IH

35 SVR

D SB

N IH

35 SVR

D N

B

S IH

35

SVR

D N

B

LE GRANDE AVE

JEWELL ST

NELLIE ST

W JAMES ST

W JAMES ST

W GIBSON ST

EVA

DR

S 5T

H S

T

S 2N

D ST

DAW

SON

RD

BRA

ZOS

ST

EAST

AVE

RA

INEY

ST

S 3R

D ST

BRU

SHY

STSA

N M

AR

CO

S ST

S L

AM

AR BL

VD

SB

SABI

NE

ST

BRA

ZOS

ST

DAVIS ST

SPENCE ST

RIVER ST

W ELIZABETH ST

RED

RIV

ER ST

DRISKILL ST

W CESAR CHAVEZ STE 2ND ST E 2ND

ST

E 3RD ST

ME

LISS

A LN

EVA

ST

E 4TH ST

LAMBIE ST

DAW

SON

RD

THE

CIRCLE

WILLOW ST

EBONY ST

W 2ND ST

HAYWOOD

AVE

E 2ND ST

RAV

INE

DR

RETAMA ST

HASKELL ST

S IH 35 SB

S 1S

T ST

S IH 35 N

B

LAMBIE ST

S IH

35

SVR

D SB

E 3RD ST

DANIEL DR

SUNNY LN

RA

INEY

ST

W RIVERSIDE

DR

LE

E BA

RTO

N DR

S C

ON

GR

ESS

AVE

N IH

35 SB

E RIVERSIDE DR

W RIVERSIDE

DR

W RIVERSIDE DR

N IH

35 SV

RD

NB

WA

LLER

ST

N IH

35

SVR

D S

B

S 3R

D ST

S 3R

D S

T

EAST

AVE

EAST

AV

E

EDGECLIFF

TER

BIC

KLE

R RD

NEWNIN

G AVE

434 432

428

432

430

432

434432

432434

432

432

430

434

428

430

434

434

432

430

434

434


Town Lake Rowing Center


0 250 500

Feet

Legend

ELEVATION

428

430

432

434


Yes

No



±Lady Bird Lake Trail Problem Flood Areas-Central

05 April 2013 MS





BlunnCreek Park

LonghornShores atTown Lake

Manuel andRobert

Donley Park

InternationalShores atTown Lake

Lakeshore atTown Lake

NorwoodTract at

Town Lake

Metz

Holly Shoresat Town Lake

Peace Pointat Town Lake

WallerBeach at

Town Lake

Central MaintenanceComplex atTown Lake

Roy G. GuerreroColorado

River Park


Festival Beach

E CESAR CHAVEZ ST

AN

THO

NY

ST

WOODLAND AVE

RIVERVIEW ST

E 2ND ST

N IH

35 SVR

D N

B

SAN

SABA

ST

S IH

35 SV

RD

NB

CANTERBURY ST

LUPINE LN

S IH

35 SV

RD

SBGARDEN ST

TAYLOR

GAINES ST

HOLLY ST

CO

MA

L ST

EAST

AVE

BELL

AIRE

DR

REAGAN

TER

E RIVERSIDE DR

SAN

SABA

ST

LOM

A DR WATERLOO

CITY LN

N PL

EASA

NT

VALL

EY R

D

E RI VERSIDE

DR

JULI

US

ST

PED

ERN

ALE

S ST

SHORE DISTRICT DR

SALI

NA

ST

LYN

N ST

LYN

N S

T

LLA

NO

ST

MIL

DR

ED S

T

N PL

EASA

NT

VALL

EY R

D

LOM

A DR

CLA

RA

STLYN

N ST

SUNNY VALE ST

GARDEN ST

TINN

IN F

ORD

RD

HOLLY ST

RIVERVIEW ST

HASKELL ST

HASKELL ST

ELMONT DR

LAD

Y BI

RD L

N

VIO

LET

CRO

WN

LNTO

W

N

LAK

E CIR

BETT

Y JO

DR

TOW

N CR

EEK

DR

JESSE E SEGOVIA ST

N IH

35 NB

UPLA

ND

DR

ROYA

L CREST DR

S PL

EASA

NT VA

LLEY

RD

PARK

ER L

N

S LAKESHORE BLVDS IH

35 SB

N IH

35 SB

E RIVERSIDE

DR

WA

LLER

ST

ELMHURST DR

E RIVERSIDE

DR

NASH HERNANDEZ SR RD

E RIVERSID E

DR

E RIVERSID

E

DR

4 3 2

430

428

434

430

434

432

432

434

434

432

434

434

434

430

428

434

432

432

428

430

432

430

430

430

428

428

428

432

432

428

430432

432

0 250 500

Feet

Legend

ELEVATION

428

430

432

434


Yes

No



±Lady Bird Lake Trail Problem Flood Areas-East End

05 April 2013 MS






Appendix C Labyrinth Weir Opinion of Probable Construction Cost

ITEM DESCRIPTION QUANTITY UNIT UNIT PRICE TOTAL

1 Mobilization 1 LS 1,058,000$ 1,058,000$

2 Temp. Erosion and Sediment Control 1 LS 75,000$ 75,000$

3 Site Preparation and Restoration 1 LS 100,000$ 100,000$

4 Lift Gate/Hoist Removal 1 LS 250,000$ 250,000$

5 Care Of water 1 LS 1,200,000$ 1,200,000$

6 Flood Insurance 1 LS 60,000$ 60,000$

7 Labyrinth Weir Spillway Slab (3 foot thick) 4,000 CY 400$ 1,600,000$

8 Labyrinth Weir (3 foot thick walls) 6,000 CY 850$ 5,100,000$

9 Concrete Apex Piers 600 CY 800$ 480,000$

10 Concrete Training Walls/ Pier Modifications 1 LS 75,000$ 75,000$

11 Debris Boom 550 LF 600$ 330,000$

12 Access Ramp 1 LS 400,000$ 400,000$

13 Access Drive Behind Labyrinth 1300 CY 400$ 520,000$

13 Additional Foundation Slab Behind Bascule Gates 980 CY 400$ 392,000$

14 Outlet Gate 1 LS 250,000$ 250,000$

15 Handrail 1 LS 41,000$ 41,000$

14 Embankment Drain Pipe 1 LS 50,000$ 50,000$

SUBTOTAL: 11,890,000$

PROJECT SUBTOTAL: 11,890,000$

CONTINGENCY: ~26% 3,110,000$

15,000,000$

IT IS NOT TO BE USED FOR CONSTRUCTION OR BIDDING PURPOSES.

THIS DOCUMENT IS RELEASED FOR PURPOSE OF INTERIM REVIEW.

AustinEnergy

OPINION OF PROBABLE CONSTRUCTION COST

PreliminaryOpinionofProbableConstructionCostLonghornDam


Prepared by Freese and Nichols, Inc.6/21/2013


Appendix D Survey Points

Austin Energy SURVEYING 2526 Kramer Ln Bldg. E Austin TX 78758-4007 1-512-505-7183 Wednesday, May 15, 2013 2:45:08 PM PROJECT: \\Kramer3\Survey\Surveying\HIKE&BIKE TRAIL SHOOTS 5-14-2013.pro -------------------------------------------------------------------------------- Point Coordinates Listing - WEST POSITIVE LAT LONG IN DECIMAL DEGREE FORMAT PT# LATITUDE LONGITUDE Elev Name ------------------------------------------------------------------------------- 1 30.252534291 97.740769389 430.63 GD 2 30.252667285 97.740596028 428.63 TOPWATER 3 30.252216609 97.740828702 434.11 PARKING LOT 4 30.252769559 97.741119026 437.87 PATIO 5 30.261354281 97.747342391 431.11 TRAIL 6 30.261364772 97.747339430 429.00 TOPWATER 7 30.261274656 97.747141271 430.86 TRAIL 8 30.262441033 97.749103028 431.47 TRAIL 9 30.262676478 97.749549873 430.61 TRAIL 10 30.262737220 97.749583191 430.23 TOPROCK 11 30.262746759 97.749582574 428.71 TOPWATER 12 30.262805644 97.749931085 430.65 TRAILTOP 13 30.262883541 97.750183914 430.19 TRAIL 14 30.262994515 97.750532940 430.63 TRAIL 15 30.263253915 97.751156217 432.44 TRAIL 16 30.263388586 97.751820995 434.65 TRAIL 17 30.263778547 97.752469411 432.37 TRAIL 18 30.263832294 97.752422993 428.86 TOPWATER 19 30.263978532 97.753395432 435.88 TRAIL 20 30.264268106 97.755176605 436.29 TRAIL 21 30.265191153 97.757224116 428.71 TOPWATER 22 30.266002982 97.755469881 447.70 STEPS-100FLOOD 23 30.266212010 97.755775002 433.00 TRAIL 24 30.266348489 97.756175346 432.54 TRAIL 26 30.266446816 97.756252431 431.94 MANHOLE 27 30.266451488 97.756247316 431.06 FLOWLINE 28 30.266310672 97.756163568 429.01 WATER EDGE 29 30.266452189 97.755365044 441.68 COA BRASS CAP 30 30.265990854 97.755067104 435.76 TRAIL 31 30.265009205 97.752369384 428.78 TOPWATER 32 30.264198293 97.750286293 432.78 TRAIL 33 30.264289096 97.750597795 432.50 TRAIL 34 30.270332016 97.765928609 428.77 TRAIL 35 30.270305007 97.765911136 429.64 TRAIL 36 30.270341835 97.765982296 430.79 TRAIL 37 30.270260260 97.765996991 428.93 TOPWATER 38 30.272001870 97.768865476 430.88 TRAIL 39 30.271554570 97.767987206 431.38 TRAIL 40 30.271083922 97.767333331 431.99 TRAIL 41 30.269136697 97.762640714 433.20 TRAIL 42 30.260833457 97.742371933 432.23 TRAIL 43 30.260672182 97.742453922 429.92 DOCK

44 30.260512663 97.741830687 435.52 BLDG FLOOR 45 30.260557416 97.741557783 451.58 DECK 46 30.253304936 97.738495525 431.14 TRAIL 47 30.255007269 97.739862665 431.49 TRAIL 48 30.252494773 97.737448214 430.43 TOPBOATRAMP 49 30.252464421 97.737472039 429.35 BOATRAMP 50 30.252447892 97.737485463 428.63 TOPWATER 51 30.252524840 97.737555622 430.49 TRAIL 52 30.252414325 97.737393241 431.15 TRAIL 53 30.250931518 97.734717255 431.08 TRAIL 54 30.250581705 97.733561912 432.27 TRAIL 55 30.250312580 97.732835691 430.62 TRAIL 56 30.249942989 97.732045777 432.00 TRAIL 57 30.249101055 97.730054305 431.80 TRAIL 58 30.248749013 97.728838213 430.29 TRAIL 59 30.248369389 97.727815986 430.06 BOATRAMP 60 30.248340644 97.727829351 428.67 TOPWATER 61 30.250622235 97.715423291 432.15 TRAIL 62 30.250625595 97.716035598 432.28 TRAIL 63 30.250575816 97.716127662 428.67 TOPWATER 64 30.244738580 97.716507037 432.35 TRAIL 65 30.244514469 97.716485861 434.48 TRAIL 66 30.244076989 97.716514240 431.84 TRAIL 67 30.244101151 97.716525271 428.72 TOPWATER 68 30.245343107 97.722504528 433.36 TRAIL 69 30.246063247 97.722195471 441.19 TRAIL 70 30.246430586 97.720699998 432.35 TRAIL 71 30.246465291 97.720676732 431.81 TRAIL 72 30.257720801 97.743123847 432.55 TRAIL


Appendix E Rating Curves

LABYRINTH WEIR DESIGN No Approach Velocity

PROJECT: Longhorn Dam TIME: 14:51:55PROJECT NO. 1 DATE: 31-May-13

FLOOD CRITERIA: PMF BY: DGM

USER INPUT

Max. Res Zr 454.0 ft ThicknessCrest el. Zc 427.6 ft Wall Tw 3 ftFloor el. Zf 403.0 ft Slab Ts 3 ftSpillway width Ws 50.0 ft Cutoff DepthApex Width 2a 2 ft Sheet Pile Ds 0 ftNo. of cycles n 2 Conc Wall Dc 4 ftMagnification L/W 4.95

LABYRINTH DIMENSIONS (Per Cycle)CHECK ON RATIOS Wall Height P 24.56341767 ft

Ld/B = 1.60 USE FEWER CYCLES Width W 25.00 ft

Ho/P = 1.08 WARNING! H/P > 0.7! Length L 123.75 ft

L/W RATIO IS OK Wall Length B 59.88 ft

Note: Ld/B must be <= 0.35 Depth D 58.95 ft

Ho/P must be <= 0.9 Head max H 26.44 ft must be >= 6 deg Wall Angle 10.10 deg

Length of Lb 95.69

Interference

Labyrinth Rating Curve Lower Crest.xls 1

Gated Spillway Labyrinth Tailwater

Discharge

(cfs)

Elevation

(ft‐msl)

Discharge

(cfs)

Elevation

(ft‐msl)

Discharge

(cfs)

Elevation

(ft‐msl)

0 428.25 0 427.56 0 410.00

37,519 428.25 3,500 428.25 14,564 416.90

45,965 429.63 5,661 428.5 24,567 421.30

54,291 431.74 8,180 428.75 29,945 423.90

63,814 433.86 14,162 429.25 50,050 430.10

73,067 435.77 21,230 429.75 90,046 438.60

82,483 437.81 29,211 430.25 90,361 438.70

91,536 439.55 37,947 430.75 366,912 459.30

101,896 440.41 47,291 431.41 402,395 460.80

51,169 431.88

55,114 432.37

59,118 432.95

63,174 433.62

67,273 434.37

71,410 435.22

75,576 436.14

79,765 437.01

83,971 437.90

88,188 438.78

92,410 439.43

96,631 439.77

100,846 440.11

105,050 440.44

109,239 440.78

113,408 441.12

117,554 441.45

121,672 441.78

125,761 442.11

129,815 442.44

405

410

415

420

425

430

435

440

445

0 25000 50000 75000 100000 125000

Gated Spillway

Labyrinth

Tailwater


Appendix F Project Schedules

ID Task Name Duration

1 Notice to Proceed 0 days

2

3 Final Design 180 days

4 30% Plans 8 wks

5 30% Review 4 wks

6 60% Plans and Specifications 8 wks

7 60% Review 4 wks


9 90% Review 4 wks

10

11 PERMITTING 85 days

12 Prepare Permit Documents 10 days

13 Finalize Plans and Specifications 2 wks

14 Finalize Report / Summary 1 wk

15 Finalize Application 1 wk

16 Permit Review (General) 75 days

17 Completion Check (no appt) 1 wk

18 Permit Submittal (appt required) 1 wk

19 Permit Review 4 wks

20 Response to Comments 2 wks



23 Finalize Permit 2 wks

24 PROJECT SETUP (COA) 10 days

25 Trade Summary 1 wk

26 COA Ecparis Setup 1 wk

27

28 Bid and Award 75 days

29 Finalize Front End 1 wk

30 CLMD Review 2 wks

31 Advertise 3 wks

32 Bid Opening 1 wk

33 Bid Evaluation 1 wk

34 Request Council Action 6 wks

35 Award 1 wk

36

37 Construction 15 mons

Month ‐2 Month 1 Month 3 Month 5 Month 7 Month 9 Month 11 Month 13 Month 15 Month 17 Month 19 Month 21 Month 23 Month 25 Month 27Quarter 1 Quarter 3 Quarter 5 Quarter 7 Quarter 9

Task

Split

Milestone

Summary

Project Summary

External Tasks

External Milestone

Inactive Task

Inactive Milestone

Inactive Summary

Manual Task

Duration‐only

Manual Summary Rollup

Manual Summary

Start‐only

Finish‐only

Deadline

Progress

Longhorn DamGate Repair Schedule

Page 1

Project: 2013‐04‐11 Longhorn DaDate: Jun 20 '13

ID Task Name Duration

1 Notice to Proceed 0 days

2

3 Preliminary Design 220 days

4 Develop Preferred Layout Based on Stakeholder Input

8 wks

5 Sectional Physical Model Study 6 wks

6 Geotechnical Investigation 10 wks

7 20% Drawings 3 wks

8 Full Width Model Study 16 wks

9 Backwater Model 2 wks

10 Preliminary Design Report (draft) 1 wk

11 City Review 4 wks

12 Preliminary Design Report (final) 2 wks

13 City Authorizes Final Design 4 wks

14

15 Final Design 170 days

16 30% Plans 6 wks

17 30% Review 4 wks


19 60% Review 4 wks


21 90% Review 4 wks

22

23 PERMITTING 85 days

24 Prepare Permit Documents 10 days

25 Finalize Plans and Specifications 2 wks

26 Finalize Report / Summary 1 wk

27 Finalize Application 1 wk

28 Permit Review (General) 75 days

29 Completion Check (no appt) 1 wk



32 Response to Comments 2 wks



35 Finalize Permit 2 wks

36 PROJECT SETUP (COA) 10 days

37 Trade Summary 1 wk

38 COA Ecparis Setup 1 wk

39

40 Bid and Award 75 days

41 Finalize Front End 1 wk

42 CLMD Review 2 wks

43 Advertise 3 wks

44 Bid Opening 1 wk

45 Bid Evaluation 1 wk

46 Request Council Action 6 wks

47 Award 1 wk

48

49 Construction 15 mons

Month ‐3 Month 1 Month 4 Month 7 Month 10 Month 13 Month 16 Month 19 Month 22 Month 25 Month 28 Month 31 Month 34 Month 37Quarter 1 Quarter 3 Quarter 5 Quarter 7 Quarter 9 Quarter 11 Quarter 13

Task

Split

Milestone

Summary

Project Summary

External Tasks

External Milestone

Inactive Task

Inactive Milestone

Inactive Summary

Manual Task

Duration‐only

Manual Summary Rollup

Manual Summary

Start‐only

Finish‐only

Deadline

Progress

Longhorn DamLabryinth Weir Schedule

Page 1

Project: 2013‐04‐11 Longhorn DaDate: Jun 20 '13